basic low Poly Illustrations mountains
You need to turn on Proportional Editing:

From the Blender Manual:

Proportional Edit is a way of transforming selected elements (such as vertices) while having that transformation affect other nearby elements. For example, having the movement of a single vertex cause the movement of unselected vertices within a given range. Unselected vertices that are closer to the selected vertex will move more than those farther from it (i.e. they will move proportionally relative to the location of the selected element
Influence
You can increase or decrease the radius of the proportional editing influence with the mouse wheel WheelUp, WheelDown or PageUp, PageDown respectively. As you change the radius, the points surrounding your selection will adjust their positions accordingly.